How can I apply a texture to a single mesh face?
In vertex edit mode, mark seams, uv project, add texture, use the uvmap setting.  
Seams are difficult to see.  They are necessary to get separation of faces for free movement of the image plane off the uv image editor
In uv editor only the target face should remain on the image area, all other faces can be moved to the side
Material Texture .. Image Mapping Extension must be &lt;&lt; Clip >>
In the images below a cube has been modified and the target face is shaped into a trapezoid.  The base color is red.   Only one face has the texture.